In recent years a trend has developed to produce products which are so called “environmentally friendly”, one aspect of which is biodegradability. To this end various bodies, such as ISO and the OECD have developed test methods to quantify biodegradability. One such test is the CO2 evolution test method, also known as the modified Sturm OECD method 301B, which test for ready biodegradability. In terms of this test, compounds can be considered to be readily biodegradable if they reach 60% biodegradation within 28 days.
Currently available middle distillates, typically crude oil derived diesel fuels, such as US 2-D grade (low sulphur No. 2-D grade for diesel fuel oil as specified in ASTM D 975-94) and/or CARB (California Air Resources Board 1993 specification) grade diesel, do not meet the biodegradability requirements of the abovementioned biodegradability test.
The prior art teaches in ZA 96/9890 that high biodegradability of hydrocarbon base oils could be derived from the presence of predominantly mono-methyl branching on the paraffinic carbon backbone. U.S. Pat. No. 5,498,596 discloses a non-toxic, biodegradable well fluid comprising 98% (mass) n-paraffins and less than 1% (mass) monocyclic aromatics as well as other olefinic components. The biodegradability of the well fluid in the US patent can not be related back to the nature of the paraffinic molecules due to the fact that biodegradability is enhanced through branching and not through linear n-paraffinic molecules. Further, WO 92/14804 discloses a low aromatic diesel fuel which comprises mainly a mixture of hydrocarbons containing not more than 1% by volume of aromatic type hydrocarbons and less than 0.05% sulfur or sulfur compounds. The fuel is disclosed as reducing unwanted emissions and improving operational performance. The disclosure however does not address the issue of biodegradeability. Still further, WO 97/14769 discloses diesel fuels having excellent lubricity, oxidative stability and high cetane number produced from the non-shifting Fischer-Tropsch process. Again no mention is made regarding biodegradability and the disclosure of oxidative stability would indicate against biodegradability.
A need thus exists for a middle distillate cut, typically a diesel fuel, which is readily biodegradable as determined by the abovementioned biodegradability test.
Surprisingly, it has now been found, that a low aromatics content and a relatively high iso-paraffins to n-paraffins ratio contribute to ready biodegradability of middle distillates, such as diesel fuel.